Platinum Group Metals

(or platinum metals), chemical elements of the second and third triads of Group VIII of the Mendeleev periodic system. The group includes the light metals ruthenium (Ru), rhodium (Rh), and palladium (Pd), with a density of about 12 g/cm³, and the heavy metals osmium (Os), iridium (Ir), and platinum (Pt), with a density of about 22 g/cm³. The platinum metals are silver-white and refractory. In addition to silver and gold, the platinum metals are called noble metals because of their attractive appearance and high chemical stability.

History. There are indications that native platinum was known in antiquity in Egypt, Ethiopia, Greece, and South America. In the 16th century the Spanish conquistadores discovered a very heavy, dull-white, infusible metal in South America together with native gold. The Spanish named it platina, which is the diminutive of plata, “silver.” In 1744 the Spanish explorer Antonio de Ulloa brought samples of platinum to London. The samples aroused great interest among European scientists. Platinum, which at first was considered white gold, was recognized as a separate metal in the mid-18th century.

In 1803 the English scientist W. H. Wollaston discovered palladium and rhodium in native platinum. Palladium was named for the asteroid Pallas, discovered in 1802, and rhodium for the pink-red color of its salts (from the Greek rhodon, “rose”). In 1804 the English chemist Smithson Tennant discovered two more metals in the residue after native platinum was dissolved in aqua regia. One of the metals came to be called iridium because of the variety of colors of its salts (from the Greek iris, genitive iridos, “rainbow”), and the other was called osmium because of the sharp odor of its tetroxide (from the Greek osme, “odor”). In 1844, while studying the refining residue of native platinum from the Urals at the St. Petersburg Mint, K. K. Klaus discovered yet another platinum metal. This new metal was called ruthenium (from the Late Latin Ruthenia, “Russia”).

Occurrence. Platinum group metals are among the rarest elements. Their average content in the earth’s crust has not yet been established precisely; approximate values are given in Table 1. The rarest metals are rhodium and iridium (1 × 10^–7 percent by weight), and the most common is osmiun (5 × 10^–6 percent). The content of platinum group metals is higher in ultrabasic and basic igneous rocks, whose origin is associated with plutonic magmatic processes. Deposits of platinum group metals are confined to such rocks. An even higher average content of platinum group metals is found in stony meteorites, which are considered analogues of the middle mantle of the earth (the content of platinum group metals in stony meteorites ranges from ten-thousandths to hundred-thousandths of a percent by weight). The native state is characteristic for platinum group metals in the earth’s crust, although a limited number of compounds of rhodium, palladium, osmium, and platinum with sulfur, arsenic, and bismuth are also known. About 30 minerals of platinum group metals have been found, of which the most are known for palladium (13) and platinum (nine). All these minerals were formed at great depths and high temperatures and pressures. Platinum and other platinum group metals are encountered as impurities in many sulfides and silicates of ultrabasic and basic rocks.

The geochemistry of platinum group metals in the biosphere has been little studied; their content in the hydrosphere and in living matter has not been established. Some sedimentary manganese ores are rich in platinum (up to 1 × 10^–3 percent); concentrations of 1 × 10^–6 percent platinum and palladium have been observed in coals. A high content of platinum group metals has been observed in Viatka phosphorites and in the ash of trees growing on platinum deposits.

Physical and chemical properties. The physical and mechanical properties of platinum metals are compared in Table 1. (It should also be noted that ruthenium and osmium are very hard and brittle, possibly because of the presence of impurities.) Rhodium and iridium are less hard and brittle, and palladium and platinum are malleable and suitable for rolling, drawing, and stamping at room temperature. The ability of ruthenium, palladium, and platinum to absorb hydrogen is of interest. This property is especially characteristic of palladium, one volume of which absorbs up to 900 volumes of H2. After absorbing such quantities of hydrogen, palladium retains its metallic appearance but cracks and becomes brittle. All the platinum group metals are paramagnetic. The magnetic susceptibility x_s × 10 ^–6 electromagnetic units at 18°C is 0.05 for osmium, 0.50 for ruthenium, 5.4 for palladium, and slightly more than 1.0 for rhodium, iridium, and platinum.

According to long-established tradition, the platinum group metals are customarily placed in Group VIII of the periodic table. Thus, all platinum metals should be expected to have a highest oxidation state of + 8. However, the + 8 oxidation state is observed only for ruthenium and osmium. Other platinum metals do not have oxidation states exceeding + 6; this is because the 4f and 5f inner subleveis in the ruthenium and osmium atoms, respectively, remain unfilled. Thus, excitation is possible not only of sublevels 5s and 6s to sublevels 5p and 6p but also of sublevels 4d and 5d to sublevels 4f and 5f. As a result, there are eight unpaired electrons and a valence of +8 in atoms of ruthenium and osmium. The electron configurations of rhodium, iridium, palladium, and platinum do not admit such a possibility. Therefore, in some versions of the Mendeleev table, these elements (as well as cobalt and nickel) are removed from Group VIII. All platinum group metals readily form complexes, in which they have various oxidation states and coordination numbers. The complexes are usually colored and very stable.

The chemical properties of the platinum group metals have much in common. They all have low reactivity in massive form, except for osmium. However, in the form of “blacks” (highly dispersed powders), platinum metals easily adsorb sulfur, halogens, and other nonmetals. (The blacks are usually obtained by reduction of platinum metals from aqueous solutions of their compounds.) Massive ruthenium, rhodium, osmium, and iridium alloyed with platinum, zinc, lead, and bismuth pass into solution under the action of aqua regia, but aqua regia does not act on these metals taken separately.

The platinum group metals may be divided into three diads, consisting of a light and a heavy metal that are located one above the other: ruthenium and osmium, rhodium and iridium, and palladium and platinum.

Upon heating with O₂ and strong oxidizing agents, ruthenium and osmium form low-melting tetroxide crystals (orange RuO₄ and yellowish OSO₄). Both of these compounds are volatile, and their vapors have an unpleasant odor and are very poisonous. Upon the action of reducing agents, they are converted to the lower oxides RuO₂ and OsO₂ or to the metals. With bases Ruo₄ forms ruthenates—for example, potassium ruthenate—by the reaction

RuO₄ + 2KOH = K₂RuO₄ + ½O₂ + H₂O

Under the action of chlorine, K₂RuO₄ + ½Cl₂ = KRuO₄ + KCl

Osmium tetroxide with KOH yields the complex K₂[[O_sO₄-(OH)₂]. With fluorine and other halogens, ruthenium and osmium react easily upon heating to form compounds of the type RuF₃, RuF₄, RuF₅, and RuF₆. Osmium gives the analogous compounds except for OsF₈; the existence of OsFe has not been confirmed. Of considerable interest are the complexes of ruthenium with xenon (Xe[RuF₆], discovered by the Canadian chemist N. Bartlett in 1962), as well as with molecular nitrogen ([(NO)(NH₃)₄NN₂Ru(NH₃)₄NO]Cl, discovered by the Soviet chemist N. M. Sinitsyn in 1962, and [Ru(NH₃)₅ N₂]Cl₂, discovered by the Canadian chemist A. D. Allen in 1965).

Aqua regia does not attack massive rhodium and iridium. The oxides Rh₂O₃ and Ir₂O₃, and Ir₂O₃, which decompose at high temperatures, are formed upon roasting in O₂.

Palladium is readily dissolved by heating in HNO3 and concentrated H2SO 4, with the formation of the nitrate Pd(NO₃)₂ and the sulfate PdSO₄. These acids do not attack platinum. Aqua regia dissolves palladium and platinum, with the formation of the complex tetrachloropalladic(II) acid, H₂[PdCl₄], and hexachloroplatinic(IV) acid, H₂[[PtCl₆]· 6H₂ O (brownish red crystals). Of the salts of hexachloroplatinic acid, ammonium chloroplatinate(IV), (NH₄)₂[PtCl₆], which forms light yellow crystals, has the greatest importance in the industry of the platinum group metals. It is only slightly soluble in water and virtually insoluble in concentrated solutions of NH4CI. Upon roasting, the salt decomposes by the reaction

In this reaction, platinum is obtained in finely divided form (platinum sponge).

Production. The isolation of platinum group metals and their production in pure form are very complex as a result of the great similarity of the metals’ chemical properties. The processes require a large expenditure of labor, time, and expensive reagents. To produce pure platinum, the initial materials—native platinum, platinum slimes (heavy residue from the washing of platinum-bearing sands), and scrap (items made of platinum or platinum alloys that are unsuitable for use)—are treated with aqua regia with heating. Platinum, palladium, and to a certain extent rhodium and iridium pass into solution as the complexes H₂[PtCl₆], H₂[PdCl₄], H₃[RhCl₆], and H₂[IrCl₆]. Iron and copper also pass into solution as FeCl₃ and CuCl₂. … The residue, which is insoluble in aqua regia, consists of iridosmine, chromite (FeCrO₂), quartz, and other minerals.

Platinum is precipitated from solution by ammonium chloride in the form of (NH₄)₂[IrCl₆]. To prevent the precipitation along with platinum of iridium in the form of the analogous insoluble complex (NH₄)₂[[IrCl₆] (the other platinum metals are not precipitated by NH₄CI), Ir(+4) is first reduced to Ir(+3)—for example, by the addition of the sugar C12H22O11 (the method of I. I. Cherniaev). The resultant compound, (NH₄)₃ [IrCl₆], is soluble and does not contaminate the precipitate.

The ammonium chloroplatinate is filtered off, washed with a concentrated solution of ammonium chloride (in which the precipitate is virtually insoluble), dried, and roasted. The platinum sponge obtained is pressed and then melted in an oxyhydro-gen flame or a high-frequency electrical furnace. Other platinum group metals are extracted from iridosmine as well as from the filtrate remaining after the precipitation of (NH₄)₂[PtCl₆] by a series of complex chemical operations. For example, to convert the platinum metals and iridosmine, which are insoluble in aqua regia, to a soluble state, the materials are roasted with the peroxides BaO₂ or Na₂O₂. Chlorination by heating a mixture of platinum concentrates with NaCl and NaOH in a stream of chlorin is also used.

Refining produces the poorly soluble complexes ammonium hexachlororuthenate, (NH₄)₃[RuCl₆]; osmyltetrammine dichloride, [OsO₂((NH₃)₄]Cl₂; chloropentamminerhodium(III)-chloride [Rh(NH₃)₅Cl]Cl₂; ammonium hexachloroiridate (IV), (NH₄)₂[IrCl₆]; and dichlorodiamminepalladium, [Pd-(NH₃)₂] Cl₂. The platinum metals are obtained as sponges by roasting the abovementioned complexes in a hydrogen atmosphere, for example,

[OsO₂(NH₃)₄]Cl₂+ 3H₂ = Os + 2H₂O + 4NH₃ + 2HCl

[Pd(NH₃)₂ cl₂ + H₂ = Pd + 2NH₃ + 2HCl

The sponge platinum metals are melted in a high-frequency vacuum electrical furnace.

Other means of refining are also used, particularly methods based on the use of ion-exchange resins.

The main sources for the production of platinum metals are sulfide copper-nickel ores, deposits of which are found in the USSR (Noril’sk and the Krasnoiarsk Krai), Canada (Sudbury district, Province of Ontario), and the Republic of South Africa. As a result of complex metallurgical treatment of the ores, the noble metals pass into “crude” metals, impure nickel and copper. Platinum group metals accumulate almost entirely in crude nickel; silver and gold, in crude copper. Upon subsequent electrolytic refining, silver, gold, and platinum group metals precipitate to the bottom of the electrolyte bath in the form of a slurry, which is then refined.

Use. Platinum is the most widely used of the platinum group metals. Until World War II, more than 50 percent of the platinum produced was used in making jewelry. In the last 20–30 years, about 90 percent of the platinum produced has been used for scientific and industrial purposes. Platinum laboratory items such as crucibles, beakers, and resistance thermometers are used in analytical and physicochemical research. About 50 percent of the platinum consumed (partially in the form of alloys with rhodium, palladium, and iridium) is used as catalysts in the production of nitric acid by oxidation of NH₃, and also in the petrochemical industry. Platinum and its alloys are used in the manufacture of apparatus for several chemical processes. About 25 percent of the platinum produced is consumed in electrotech-nology, radio engineering, automation, remote-control technology, and medicine. Platinum is also used as an anticorrosion coating.

Iridium is mainly used in the form of a 10-percent alloy with platinum. This alloy was used for the production of the international standard meter and kilogram. It is also used in the manufacture of crucibles in which crystals suitable for laser technology are grown and in the production of contacts for especially critical subassemblies in weak-current technology. Supports for compass pointers are made of an iridium-osmium alloy.

Ruthenium has the ability to accumulate H 2 and catalyze many chemical reactions; it is a constituent of several alloys that have great hardness and resistance to abrasion and oxidation.

Rhodium, because of its ability to reflect about 80 percent of visible light, and also its high oxidation resistance, is a good material for coating the reflectors of searchlights and the mirrors of precision instruments. However, its main use is in alloys with platinum, from which laboratory and industrial apparatus and wire for thermoelectric pyrometers are made.

Palladium black is used primarily as a catalyst in many chemical processes, particularly hydrogenation. Palladium is used in the manufacture of jewelry. A solution of H₂[PdCl₄] is a sensitive reagent for identifying carbon monoxide. A paper strip impregnated with this acid blackens at an atmospheric CO concentration of 0.02 mg per liter because of the release of palladium as a black in the reaction

H₂[PdCl₄] + H₂O+ CO = 4HC1 + CO₂ + Pd

The refining of platinum group metals is accompanied by the liberation of poisonous Cl₂ and NOCI, which necessitates good ventilation and possibly the sealing of the reaction apparatus. Vapors of the volatile RuO₄ and OSO₄ lead to general poisoning, as well as serious injury to the respiratory tract and the eyes, even involving loss of vision. When these compounds come in contact with the skin, it darkens (because of the reduction of the tetroxides to RuO₂, OsO₂, or metallic ruthenium or osmium) and becomes inflamed. Sores that heal with difficulty may also form. Safety measures include good ventilation, the use of rubber gloves and protective glasses, and the absorption of RuO₄ and OsO₄ vapors by solutions of alkalies.

Platinum group metals in the organism. In living organisms the platinum group metals are represented mainly by ruthenium but also by artificial radioisotopes of ruthenium and rhodium. Saltwater and freshwater algae concentrate radioisotopes of ruthenium by factors of 10²—10³ relative to the environment; crustaceans, by factors of 10–10²; mollusks, by factors up to several tens; and fishes and tadpoles, by factors from unity to 10². The isotope ¹⁰⁶ Ru migrates rapidly in the soil and accumulates in the roots of terrestrial plants. In terrestrial mammals, radioisotopes of ruthenium are taken up through the digestive tract, penetrate the lungs, and are deposited in the kidneys, liver, muscles, and skeleton. The radioisotopes of ruthenium are a component of radioactive pollution of the biosphere.